Prostaglandins but not nitric oxide are endothelium-derived relaxing factors in the trout aorta

yAIM: To identify the type of prostanoids produced by endothelial cells of trout aorta and to determine whether or not the smooth muscle responds to n itric oxide. METHODS: Ventral aortas, with and without endothelium from rai nbow trout (S gairdneri), were incubated in a buffered salt solution. RESUL TS: Addition of the calcium ionophore A23187 caused a significant increase in prostaglandin E's and a consistent increase in the stable metabolite of prostacyclin (6-keto-prostaglandin F-1 alpha) in the incubation media only when the endothelium was present. This production was inhibited by methylen e blue (10 mu mol/L). In rings of trout aorta without endothelium suspended for the measurement of isometric force in organ chambers, prostacyclin and prostaglandin E-1 but not prostaglandin E-2 caused concentration-dependent decreases in tension when the rings were contracted with acetylcholine. Th e smooth muscle did not relax to nitric oxide but did so to sodium nitropru sside. Relaxations to the latter nitrovasodilator were not inhibited by met hylene blue. Descending aorta without endothelium from frogs relaxed in a c oncentration-dependent manner to nitric oxide. CONCLUSION: Predominant endo thelium-derived relaxing factors in trout aorta are prostaglandins, the syn thesis of which can be inhibited by methylene blue. A phylogenetic appearan ce of nitric-oxide sensitive mechanism for vasodilatation, perhaps is assoc iated with the transition from water to air respiration.